1. Field of the Invention
The present invention relates to a metal material for electrode formation. More particularly, the present invention relates to a metal material for electrode formation, which can form an electrode without adopting vapor deposition, can easily realize an increase in size, can reduce production cost, and does not cause any disconnection of the electrode upon exposure to flexure and, thus, is highly reliable, and, at the same time, has a high level of electron injection function, and an organic function element using the metal material.
2. Background Art
In organic function elements, for example, organic semiconductor elements, organic thin-film transistor elements (hereinafter referred to as “organic TFT elements”) and organic electroluminescent elements (hereinafter referred to as “organic EL elements”), more charges, particularly electrons, should be injected into an organic material layer.
Substances with a low work function have a good electron injection effect, and, thus, alkali metals and alkaline earth metals are suitably used as an electron injection layer. An electron injection layer of organic EL elements and the like has hitherto been formed by stacking an alkali metal or an alkaline earth metal and a metal(s) other than these metals on top of each other usually, for example, by vapor deposition (for example, see Japanese Patent Laid-Open Nos. 320763/1997, 12381/1998, and 329746/1999).
According to the techniques disclosed in the above patent documents, an electrode is formed by vacuum deposition. For example, a plurality of types of metals are provided as independent vapor deposition sources, and an electrode region formed of an alloy containing an alkali metal or an alkaline earth metal is formed by codeposition near a luminescent layer in the element. According to another conventional technique, an alloy of an alkali metal or an alkaline earth metal with other metal is provided, and an electrode is formed by vapor deposition or sputtering using this alloy as a target material.
Alkali metals and alkaline earth metals, however, are highly oxidizable and highly combustible in the air and thus are unstable. Due to these properties, the alkali metals and alkaline earth metals are difficult to handle. For this reason, in forming an electron injection layer in the electrode, film formation should be carried out in vacuo by vapor deposition or the like. The above problem is found not only in organic EL elements but also in general organic function elements comprising an organic material layer and an electrode.
Regarding the method for EL layer formation, in general, when a low-molecular material is used as the material for an EL layer, vacuum deposition using a mask is used, while, when a polymeric material is used, methods in which a material is brought to a solution which is then coated, for example, ink jetting, spin coating, printing, and transfer methods are used. In recent years, there is a report about coatable low-molecular materials.
In mask vacuum deposition for low-molecular materials, an increase in size of a vacuum device and a deposition mask is difficult. This disadvantageously poses a problem that increasing the size of the substrate is difficult, and the mass production of large elements is difficult. This means that, although, on a scale of an experimental level in a development stage, the vacuum deposition method can be applied even in the case of a large substrate, in a full-scale mass production stage, market competitiveness is low for tact and cost reasons.
On the other hand, for polymeric materials and coatable low-molecular materials, film formation can be carried out by wet processes such as ink jetting, printing, casting, alternate absorption, spin coating, and dipping methods, and, thus, there is no problem with coping with large substrates. This renders the coating process favorable as a method for organic EL element formation. For example, as disclosed in Japanese Patent No. 3239991, an EL layer can easily be formed by dissolving PPV (polyphenylene vinylene) as a polymeric organic EL material in an organic solvent to prepare a coating liquid which is then spin coated onto a transparent electrode.
Regarding a negative electrode, since a low-work function metal such as Al (aluminum) or Ag (silver) is used, the electrode is formed as a film by vacuum deposition of these metals.
When the vacuum deposition method is applied to the formation of the negative electrode, however, in forming an EL layer by coating, a vacuum device should be provided only for the negative electrode formation. Further, when the step of vacuum deposition is included in the production process, production tact is sometimes delayed due to vacuuming. Therefore, the feature of organic EL materials which can be coated for film formation cannot be fully utilized. Accordingly, the development of metal materials for negative electrode which can be formed by coating has been desired.
Flexibility may be mentioned as other feature of an organic EL film formed by coating. When an element is formed using a flexible base material such as a resin or a plastic, the so-called “flexible element” can be produced. However, a negative electrode has been an obstacle to the formation of the flexible element. Specifically, even when the substrate and the organic material layer are flexible, the electrode of a metallic thin film which has been formed by vapor deposition as in the prior art is not flexible and, upon flexing, is disconnected.
Regarding the utilization of Ga (gallium) and a Ga alloy, which is a liquid metal, as an electrode in the broad sense, Japanese Patent Laid-Open No. 74503/1993 discloses that Ga and a Ga alloy are used in electrical connecting means. According to this technique, attention has been drawn to easy bonding and easy separation of the liquid metal, and the liquid metal is utilized, for example, in contact points of connector pins and the like. This publication, however, does not disclose the utilization of the liquid metal as an electrode for energization of an organic material layer or the action of an electric field on the organic material layer for developing the function of organic function elements.
Japanese Patent Laid-Open No. 8443/2002 discloses a heat curable metal paste prepared by mixing a Ga-base liquid metal and Ag (silver) or Cu (copper) metal powder together. This curing action utilizes a diffusional reaction, and the diffusion region in which an alloying reaction takes place is on the order of several microns. Therefore, the coating thickness of the paste should be not more than 10 μm (see Japanese Patent Laid-Open No. 326411/1994, paragraph No. 0024).
On the other hand, all of screen printing, metal mask printing, and dispenser coating are a simple and cost competitive film formation method. These coating methods, however, are not suitable for thin film formation.
Further, in forming a film by printing a metal paste, there is a fear of causing metal powder to damage the organic EL layer. In the above patent document, an Ag powder having a particle diameter of 2 μm is used. Since, however, the organic EL layer is generally an ultrathin film having a thickness of not more then 100 nm, the separation of the organic EL layer is unavoidable. The separation of the organic EL layer is causative of a fatal trouble of electrical contact between opposed electrodes. Further, as a matter of course, heat curing of the metal paste poses a problem of disconnection.
In recent years, flat displays have become used in various fields and places, and advance in information technology has rendered flat displays more and more important. At the present time, liquid crystal displays (also known as “LCDs”) are representative flat displays. The development of organic ELs, inorganic ELs, plasma display panels (also known as “PDPs”), light emitting diode displays (also known as “LEDs”), vacuum fluorescent displays (also known as “VFDs”), field emission displays (also known as “FEDs”) and the like as flat displays based on a display principle different from that of LCDs are also being energetically made.
Among them, organic ELs are particularly energetically studied. Organic EL is also known as OEL or an organic light emitting diode (OLED).
OEL elements and OLED elements have a structure comprising an organic compound-containing layer (EL layer) held between a pair of anode and cathode electrodes. According to Japanese Patent No. 1526026, Tang et al., the structure is basically a laminated structure of “positive electrode/hole injection layer/luminescent layer/negative electrode.” Further, Tang et al. uses a low-molecular material, whereas Henry et al. (Japanese Patent No. 3239991) uses a polymeric material.
Further, a hole injection layer and an electron injection layer have been used for luminescent efficiency improvement, and a fluorescent colorant or the like has been doped into a luminescent layer for controlling a luminescent color. Furthermore, in organic ELs, since high-luminance luminescence is obtained by driving at a relatively low voltage of not more than 10 V, the application of organic ELs as lighting equipment which is an alternative to fluorescent lamps involving a problem derived from the use of mercury is also expected. Further, the conventional organic EL element uses only fluorescent luminescence which takes place in return of a singlet excited state to a ground state. In recent years, phosphorescence which takes place in return of a triplet excited state to the ground state could have become effectively utilized. This had led to improved luminescent efficiency.
Here the conventional organic EL element will be simply described. FIG. 7 is a typical diagram showing a basic sectional structure of the conventional organic EL element 101. An organic EL element 101 has a basic structure comprising an organic material layer including at least a luminescent layer (EL layer) 104 held between an anode 103 and a cathode 105. An electric field is applied across both the electrodes to allow current to flow into the EL layer and thus to emit light. If necessary, the luminescent layer 104 may have a multilayered structure which has auxiliary layers such as a hole injection layer 106 or an electron injection layer 107.
The organic EL element 101 is generally produced by providing a light transparent substrate 102 such as a glass substrate or a plastic substrate, and, due to the relationship between the energy level of the EL layer and the work function of the transparent electrode such as ITO, forming, on the substrate 102, a transparent electrodes as a positive electrode 103 and then an EL layer as a luminescent layer 104 and a negative electrode 105 as a counter electrode in that order. In the organic EL element 101 having the above structure, luminescence 108 can be observed from the transparent electrode (positive electrode 103) side.
Organic functional materials including organic ELs have low resistance to oxygen and water, and, thus, element characteristics are significantly deteriorated in an air exposed state as shown in FIG. 7. Accordingly, it is common practice to adopt an element structure as shown in FIG. 8 in which a sealing body 109 is provided for shielding the assembly from the air. In FIG. 8, the sealing body 109 is fixed into the element substrate with the aid of an adhesive 110 to shield the element from the ambient air, and, further, a desiccant 111 is provided within the sealing body for deterioration preventive purposes. The opposed electrodes 103,105 are connected respectively to terminal electrodes 103,103′ so that drive signals can be applied through the terminal electrodes from the outside of the sealing body.
The transparent electrode can be provided separately from subsequent EL layer formation by sputtering or vacuum-depositing a transparent electrically conductive film such as ITO or IZO onto a transparent substrate.
As described above, the self-luminous flat display element has excellent features, but on the other hand, it suffers from problems such as difficulties of increasing the size of the substrate, difficulties of mass-producing large elements, and disconnection of electrodes upon flexing.